Selective Catalytic Reduction Catalyst Research Articles

Reducing the poisoning effect of adsorbed alkaline earth metal on Nb active sites with sulfates for NH3-SCR

Reducing the poisoning effect of alkaline earth metals over catalysts in industrial fields presents a great challenge to selective catalytic reduction (SCR) of NOx with ammonia. Herein, we successfully introduce sulfates to the surface of supported manganese-based oxides (Nb/MnO2-S) for trapping calcium species, and Nb/MnO2-S catalyst with superior low-temperature performance and calcium-resistant property still exhibits excellent de-NOx activity in a wide operating temperature window (175–350 °C, over 80 % NOx conversion with a gas hourly space velocity of 60,000 h−1) after calcium poisoning. In this case, the addition of sulfates increased the quantity of chemisorbed oxygen on Nb/MnO2-S, thus accelerating the redox cycle in NH3-SCR reaction. Significantly, compared with calcium-poisoned Nb/MnO2, relevant spectroscopy analysis and theoretical calculations further reveal that sulfates species prefer to interact with calcium and release the Nb active sites, which maintains the efficient NH3 adsorption and preserves a large amount of Brønsted acid sites over calcium-poisoned Nb/MnO2-S, thus promoting calcium-resistant performance. This work will provide a general strategy to develop calcium-resistant SCR catalysts with low-temperature activity for industrial applications.

Facile H2O-Contributed O2 Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2 Poisoning.

Mn-based catalysts preferred in low-temperature selective catalytic reduction (SCR) are susceptible to SO2 poisoning. The stubborn sulfates make insufficient O2 activation and result in deficient reactive oxygen species (ROS) for activating reaction molecules. H2O has long been regarded as an accomplice to SO2, hastening catalyst deactivation. However, such a negative impression of the SCR reaction was reversed by our recent research. Here, we reported a H2O contribution over Mn-based SCR catalysts to counteract SO2 poisoning through accessible O2 activation, in which O2 was synergistically activated with H2O to generate ROS for less deactivation and more expected regeneration. The resulting ROS benefited from the energetically favorable route supported by water-induced Ea reduction and was actively involved in the NH3 activation and NO oxidation process. Besides, ROS maintained high stability over the SO2 + H2O-deactivated γ-MnO2 catalyst throughout the mild thermal treatment, achieving complete regeneration of its own NO disposal ability. This strategy was proven to be universally applicable to other Mn-based catalysts.

Understanding enhancement of strong Copper-Yttrium interactions on catalytic properties of Cu/Y-SSZ-13 for NH3-SCR

Cu-SSZ-13 represents one of the most promising catalysts for selective catalytic reduction of NOx with ammonia (NH3-SCR), but it has remained challenging to gain superior low-temperature activity and hydrothermal stability for meeting the stringent emission regulations. Here, Cu/Yx-SSZ-13 have been prepared by in situ doping of Y and subsequent ion-exchanging with Cu ions. The combined analysis of various energy spectra techniques unveils that Y ions are mainly located in the six-membered rings (6MRs) of chabazite framework and have a strong interaction with Cu cations in 8MRs, resulting in the remarkable low-temperature NOx abatement efficiency and hydrothermal stability. Mechanism studies reveal that the interplay between Y and Cu not only enhances the electron transfer from Cu to Y ions that favors the ammonia solvation process and reduces the reaction barrier, but also inhibits the aggregation of Cu sites. This work paves the way for next generation of advanced NH3-SCR catalysts.

Glass-fiber catalysts for selective catalytic reduction of nitrogen oxides by CO and hydrocarbons

Glass-fiber catalysts (GFCs) are investigated in the process of the catalytic reduction of nitrogen oxides by hydrocarbons and carbon monoxide contained in waste and flue gases. Studies show that the platinum-based GFCs prepared by surface high-temperature synthesis approach demonstrates higher activity in the reactions of NOx reduction and CO and CxHy oxidation, in comparison with conventional GFCs and catalytic systems based on oxides of MnCe and FeV. Structured cartridges with a corrugated and flat metal mesh with longitudinal flow direction and satin type GFC were found to provide the highest apparent catalyst activity. The kinetic model for Pt/GFC was proposed based on the experimental data. The best kinetic description is provided under account of formation of nitrite-nitrate surface compounds resulting from NO chemisorption and further interaction of these compounds with CO, as well as under account of simultaneous catalytic and non-catalytic oxidation of propane by oxygen both to CO and CO2. The proposed model may adequately reproduce the non-trivial experimental regularities, such as CO and NO conversion decrease at high temperatures and synergetic effect of oxygen on the rate of NO reduction by CO.

Effect of different acid anions on highly efficient Ce-based catalysts for selective catalytic reduction of NO with NH3

Effect of different acid anions on highly efficient Ce-based catalysts for selective catalytic reduction of NO with NH3

The influence of annealing temperature on NH3-SCR performance of CexZr1-x/UiO-66 bimetal MOFs

MOFs possess large surface area and high porosity, but they will convert into derivatives when subjecting to high temperature due to its poor thermal stability. Herein, a series of Ce-Zr bimetal MOFs derivatives were prepared by directly pre-annealing of CexZr1−x/UiO-66 bimetal MOFs at different temperatures under air atmosphere. The as-prepared samples were used as catalysts for selective catalytic reduction of NOx with NH3. The catalytic activity test results indicated that pre-annealing temperature had an obvious promoting effect on NH3-SCR performance. C0.5Z0.5/U-400 catalyst achieved a NOx conversion of 92% at 350 ºC and a complete N2 selectivity in a wide temperature range of 150–500 ºC. Various characterization tools were employed to reveal the effects of pre-annealing temperature on the microstructure and chemical properties of C0.5Z0.5/U bimetal MOFs and its derivatives. The pre-annealing treatment could lead to a distinctive decrease in specific surface area, demonstrating that C0.5Z0.5/U bimetal MOFs gradually converted into derivatives and there was an obvious agglomeration phenomenon between derived metal oxides. But a relatively large specific surface area and fluffy porous microstructure could be obtained for pre-annealing C0.5Z0.5/U at 400 ºC for 2 h. It was conducive to exposure of more active sites, thus enhancing catalytic performance. Moreover, C0.5Z0.5/U-400 catalyst not only possessed a higher Ce3+/(Ce3+ + Ce4+) ratio and a better reduction ability, but also had more abundant oxygen vacancies to adsorb and activate reactant gases. In-situ DRIFTS results indicated that NH3-SCR reactions over C0.5Z0.5/U-400 catalyst followed both E-R and L-H mechanisms.

Nitration-Promoted Vanadate Catalysts for Low-Temperature Selective Catalytic Reduction of NOX with NH3.

Vanadium-based catalysts have been commercially used in selective catalytic reduction (SCR), owing to their high catalytic activity and effectiveness across a wide temperature range; however, their catalytic efficiency decreases at lower temperatures under exposure to SOX. This decrease is largely due to ammonium sulfate generation on the catalyst surface. To overcome this limitation, we added ammonium nitrate to the V2O5-WO3/TiO2 catalyst, producing a V2O5-WO3/TiO2 catalyst with nitrate functional groups. With this approach, we found that it was possible to adjust the amount of these functional groups by varying the amount of ammonium nitrate. Overall, the resultant nitrate V2O5-WO3/TiO2 catalyst has large quantities of NO3- and chemisorbed oxygen, which improves the density of Brønsted and Lewis acid sites on the catalyst surface. Furthermore, the nitrated V2O5-WO3/TiO2 catalyst has a high NOX removal efficiency and N2 selectivity at low temperatures (i.e., 300 °C); this is because NO3- and chemisorbed oxygen, generated by nitrate treatment, facilitated the occurrence of a fast SCR reaction. The approach outlined in this study can be applied to a wide range of SCR catalysts, allowing for the development of more, low-temperature SCR catalysts.

Prediction of Cu Zeolite NH3-SCR Activity from Variable Temperature 1H NMR Spectroscopy.

Selective catalytic reduction (SCR) of NOx by ammonia is one of the dominant pollution abatement technologies for near-zero NOx emission diesel engines. A crucial step in the reduction of NOx to N2 with Cu zeolite NH3-SCR catalysts is the generation of a multi-electron donating active site, implying the permanent or transient dimerization of Cu ions. Cu atom mobility has been implicated by computational chemistry as a key factor in this process. This report demonstrates how variable temperature 1H NMR reveals the Cu induced generation of sharp 1H resonances associated with a low concentration of sites on the zeolite. The onset temperature of the appearance of these signals was found to strongly correlate with the NH3-SCR activity and was observed for a range of catalysts covering multiple frameworks (CHA, AEI, AFX, ERI, ERI-CHA, ERI-OFF, *BEA), with different Si/Al ratios and different Cu contents. The results point towards universal applicability of variable temperature NMR to predict the activity of a Cu-zeolite SCR catalyst. The unique relationship of a spectroscopic feature with catalytic behavior for zeolites with different structures and chemical compositions is exceptional in heterogeneous catalysis.

Excellent alkali resistance of Fe, Ce and sulfates co-modified V2O5–WO3/TiO2 catalyst for selective catalytic reduction of NOx with NH3

Improving the alkali resistance of catalysts for selective catalytic reduction of NOx with NH3 is still a challenge. In this work, the co-modification with Fe, Ce and sulfates on V2O5–WO3/TiO2 catalysts (denoted as xSFeCeVWTi) significantly enhances its alkali resistance with K element as a representative. A series of xSFeCeVWTi catalysts was synthesized by wet impregnation with designed 0.05Fe/V, 1.5Ce/V and different S/V molar ratios x. The NOx conversion and K resistance of xSFeCeVWTi catalysts increase with the increase of loading amounts of sulfates but no longer further increase as the sulfates load is excessive to block the pores of catalysts and hinder the adsorption of reactants. The optimal modified catalyst in about 2.2S/V actual loading ratio, corresponding to 10.5SFeCeVWTi sample, shows over 99.0% NOx conversion and N2 selectivity at 300–400 °C after K-poisoning. The interaction between Fe, Ce and V improves its redox ability but slightly weakens surface acidity, while the proper amount of sulfate species enriches surface Brønsted acid sites but attenuates its redox capability. However, a balance of redox capacity and surface acidity, caused by Fe, Ce and sulfate co-modification, contributes to the excellent K resistance of 10.5SFeCeVWTi catalyst. Finally, the change of physicochemical properties influences the reaction mechanism which follows the Eley–Rideal mechanism on 10.5SFeCeVWTi catalyst. These results show that the co-modification with Fe, Ce and sulfates is a good way to improve the alkali tolerance of V2O5–WO3/TiO2 catalyst in industrial applications.

Selective leaching of V, W and recycling of TiO2 carrier from waste V2O5-WO3/TiO2 catalysts by mixed alkali liquor

Recovery of selective catalytic reduction (SCR) denitrification catalysts is important to reduce costs and environmental pollution. To recover and reuse the metal resources in the waste V2O5 - WO3/TiO2 catalyst, a mixed alkaline leaching system of NH3·H2O and sodium compounds was used. The method keeps the carrier composition and structure unchanged while achieving the separation of V and W. The effects of three additives, NaOH, Na2CO3 and NaCl, were investigated after determining the optimum leaching concentration of NH3·H2O at 6.0 mol‧L−1. The results showed the most superior leaching efficiency of V and W in NH3·H2O - NaOH solution. Next, the effects of additive concentrations and process parameters on the leaching of V and W were systematically investigated. Under optimum leaching conditions, 77.47%V and 47.7%W were leached without destroying the carrier. Moreover, this method showed similar separation of valuable metals from different components of waste catalysts, demonstrating its versatility. Meanwhile, the TiO2 carriers with undamaged structure can be directly prepared as new denitrification catalysts. Finally, the comprehensive utilization of 92.84%W, 91.94%V and 92.17%Ti resources in the waste denitrification catalyst is realized. Overall, the method may be an environmentally friendly solution for resource recovery of waste SCR catalysts.

Physicochemical properties and NH<sub>3</sub>-SCR catalytic performance of intercalated layered aluminosilicates

The representative of natural layered clays, bentonite, was modified according to two routes and tested as a new catalyst for selective catalytic reduction of nitrogen oxides with ammonia (NH<sub>3</sub>-SCR). The natural acid-activated clay was ion-exchanged with Na<sup>+</sup> or remained in H-form and pillared with metal oxides. In order to limit the number of synthesis steps, iron as an active phase was introduced simultaneously with Al<sub>2</sub>O<sub>3</sub> during the intercalation procedure. Additionally, the samples were doped with 0.5 wt% of copper to promote low-temperature activity. It was found that the performed modifications resulted in disorganization of the ordered layered arrangement of bentonite. Nevertheless, acid activation and pillaring improved structural and textural parameters. The results of catalytic tests indicated that the samples containing Fe<sub>2</sub>O<sub>3</sub> pillars promoted with Cu exhibited the highest NO conversion of 85% at 250 °C (H-Bent-AlFe-Cu) and 75% at 300 °C (Na-Bent-AlFe-Cu). What is important, activity of the protonated samples in the high-temperature region was noticeably affected by the side reaction of ammonia oxidation, correlated with the production of NO and resulting in N<sub>2</sub>O emission during the process comparing to Na-Bentonite catalysts.

A Study of CeSnOx and Pd/CeSnOx as Low-Temperature NOx Adsorbers with Excellent Hydrothermal Stability.

In the present work, we report on two passive NOx adsorber (PNA) material candidates: the novel support CeSnOx with and without Pd loading. The NOx adsorption and storage capacities of fresh and hydrothermally aged CeSnOx and Pd/CeSnOx were investigated. The results show that CeSnOx exhibits a rather large NOx uptake and storage capacity (28.9 μmol/g), while the loading of Pd on CeSnOx can further increase the storage capacity to 37.6 μmol/g and affect the desorption temperature of NOx. It was found that the NOx desorption temperature of Pd/CeSnOx was compatible with the efficient operating window of selective catalytic reduction (SCR) catalysts. After a hydrothermal aging treatment at 800 °C for 12 h, the NOx adsorption and storage capacities of CeSnOx and Pd/CeSnOx increased, indicating excellent hydrothermal stability. The interaction of Pd with CeSnOx, the state of Pd species, and the structure of CeSnOx and Pd/CeSnOx are studied by combination of the characterization results.

Effect and optimization of radial non-uniform porosity catalyst on SCR characteristics based on response surface method

Selective catalytic reduction (SCR) system is used to the denitration (de-NOx) of diesel engine exhaust after treatment. In this work, a three-dimensional SCR model with radial non-uniform distribution of catalysts is established to study the pressure drop and de-NOx performance. Firstly, the SCR converter model with uniform catalyst distribution is selected to verify its reliability. Then, the radius R1 and porosity ԑ1 of area 1, radius R2 and porosity ԑ2 of area 2 are selected as single factor variables to study the influence on the SCR system performance. Finally, response surface methodology (RSM) is employed to evaluate and optimize the above single factor parameters affecting de-NOx performance and pressure drop of SCR system. The results showed that the change of ԑ2 had the greatest impact on de-NOx efficiency and pressure drop. The optimum condition for maximum de-NOx efficiency was estimated at the R1 of 50 mm, R2 of 100 mm, porosity ԑ1 of 0.47 and porosity ԑ2 of 0.7. Under these conditions, the de-NOx efficiency by numerical simulation was 80.37 %, which was close to the predicted value (80.68 %), and the pressure drop was 218 Pa, which was close to the predicted value (246 Pa). Compared with the uniform distribution of catalysts, the de-NOx efficiency of the optimal Case of numerical simulation is increased by 3.74 %, and the pressure drop is reduced by 46 Pa. The research can provide theoretical and engineering references for the improvement of SCR catalyst coating process.

Commercial SCR catalyst modified with Cu metal to simultaneously efficiently remove NO and toluene in the fuel gas.

Developing an environmentally friendly selective catalytic reduction (SCR) catalyst to effectively eliminate both nitric oxides (NO) and toluene has garnered significant attention for regulating emissions from automobiles and the combustion of fossil fuels. This study synthesized a series of novel commercial V2O5-WO3/TiO2 catalysts modified with Cu through the wet impregnation method, which was employed to simultaneously remove NO and toluene from the fuel gas. The assessment of catalyst removal performance was conducted at a selective catalytic reduction system, and the experimental results showed a significant increase in the catalytic activity due to the modification of the copper metal. The 10% Cu/SCR catalyst showed a superior activity that the NO and toluene conversion reached 100% and 95.56% at 300 °C, respectively. Subsequently, various characterization techniques were employed to investigate the crystal phase, morphology, physical features, chemical states, and surface acidity properties of the synthesis catalysts. According to the characterization results, the presence of Cu metal did not have a noticeable impact on the physical property. However, the redox performance was enhanced, and the number of surface acidic sites was also increased after adding Cu to the SCR catalyst. Furthermore, the redox cycle of Cu metal and V species was facilitated to produce more active oxygen which helped to improve the NO and toluene conversion. This work offered a novel perspective into the synergistic oxidation of both NO and toluene, which was potentially relevant for improving the selective catalytic reduction process in coal-fired power plants.

On the monolayer dispersion behavior of Co3O4 on HZSM-5 support: designing applicable catalysts for selective catalytic reduction of nitrogen oxides by ammonia

On the monolayer dispersion behavior of Co3O4 on HZSM-5 support: designing applicable catalysts for selective catalytic reduction of nitrogen oxides by ammonia

A novel high activity MnXFe3−XO4 spinel catalyst for selective catalytic reduction of NO using NH3 prepared by a short process from natural minerals for low-temperature sintering flue gas: Effect of X value on catalytic mechanism

The process of smelting and purifying the catalyst precursor salt from minerals is extremely complex, which directly leads to high catalyst costs and serious secondary pollution. In order to achieve energy saving and emission reduction in the catalyst preparation process, in-situ synthesis of catalyst materials from natural minerals is a new research direction. In this study, we firstly explored the optimal X value of MnXFe3-XO4 for the NH3 selective catalytic reduction of NO (NH3-SCR) reaction, i.e., the Mn, Fe ratio, and then prepared a novel highly active mineral-based pure phase MnXFe3-XO4 spinel NH3-SCR catalyst by natural ferromanganese ore fines with iron-red fines (Fe2O3) allotment through in situ solid-phase synthesis and magnetic separation methods according to this ratio. The results show that the X value of 1.5 (Mn1.5Fe1.5O4) is the best for NH3-SCR reaction. Mn1.5Fe1.5O4 nano-particles (201 nm) has nearly 100 % NO conversion (with 5 % H2O(g)) at 125–300 °C. The combination of characterizations and density functional theory (DFT) calculation shows that the catalytic process of Eley-Rideal (E-R) dehydrogenation is enhanced at both the active site Mn site and Fe site, which is a key factor in the acceleration of the NH3-SCR reaction with increasing X value at the MnXFe3-XO4 surface.

Insights into the simultaneously enhanced activity, selectivity, and H2O resistance of cobalt modified MnCeOx/TiO2 catalyst for selective catalytic reduction of NOx with NH3

The promotional effect of cobalt (Co) modification on MnCeOx/TiO2 catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR) was investigated extensively in this study. A series of CoMnCeOx/TiO2 catalysts with different Co contents were prepared by the facile impregnation method and characterized by XRD, N2 sorption, XPS, H2-TPR, NH3/NO-TPD, and in situ DRIFT spectroscopy. The results indicate that the modification of MnCeOx/TiO2 by Co could significantly improve its low-temperature activity and N2 selectivity, and broaden operation temperature window (>95% NO conversion at 100–225 °C), and that the optimal mole ratio of Co to Ti in the catalyst is 0.4. Furthermore, the introduction of Co to MnCeOx/TiO2 could obviously enhance H2O resistance. The characterization results reveal that the addition of Co to the MnCeOx/TiO2 catalyst could enhance the dispersion of MnOx and CeOx, and increase the relative ratio of Mn4+ and Ce3+ on the catalyst surface. In addition, the surface area of MnCeOx/TiO2 is enlarged evidently after doping of Co. From the results of H2-TPR, NH3/NO-TPD, and NO oxidation, CoMnCeOx/TiO2 exhibits stronger reducibility, larger adsorption capacity for NH3 and NO, and higher NO to NO2 conversion than MnCeOx/TiO2, thus obtaining the higher SCR activity, N2 selectivity, and H2O tolerance. In situ DRIFT spectroscopy tests indicate that the NH3-SCR reaction on both MnCeOx/TiO2 and CoMnCeOx/TiO2 follow simultaneously the Langmuir-Hinshelwood and Eley-Rideal mechanisms.

Promoting the catalytic activity and SO2 resistance of CeO2 by Ti-doping for low-temperature NH3-SCR: Increasing surface activity and constructing Ce3+ sites

CeO2-based oxides are promising catalysts for low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), but the poor SO2 poisoning resistance hampers its further application. Herein, we developed a Ti-doped CeO2 (Ti-CeO2) catalyst with high specific surface area by calcining TiSO4-doped Ce-MOF precursor for NO removal. On Ti-CeO2, abundant Ti3+ sites are generated by the thermally reducing carbon in the organic ligand during calcination, the electronic interaction between Ti3+ and Ce4+ leads to the generation of many more exposed Ce3+ sites, effectively preventing the sulphuration of surface Ce species to Ce2(SO4)3. Besides, the residual sulfate species provides plentiful Brønsted acid sites for NH3 adsorption. Therefore, both the catalytic performance and SO2 resistance of CeO2 are significantly enhanced after Ti-doping. Remarkably, above 80% NO conversion is obtained between 220 and 450 °C on Ti0.10-CeO2 (the molar ratio of Ti:Ce is 0.10 in the synthesis process), and the NO conversion maintains around 76.0% for 170 h in the presence of 100 ppm SO2 and 5 vol% H2O at 250 °C. The reaction on CeO2 and Ti-CeO2 follow the Langmuir-Hinshelwood (L-H) mechanism, but abundant Ce4+ in CeO2 having high oxidation ability leads to the generation of plentiful stable nitrate or nitrite to occupy the active sites. In contrast, the over oxidation of NO and SO2 adsorption are inhibited because of Ti-doping, resulting in superior catalytic performance in the SCR reaction.

Mechanism study of Na poisoning on Ce/TiO2(001) surface based on adsorption of reaction species and elementary reaction pathways

The presence of alkali metal Na in flue gas can significantly impair the reactivity of Ce/TiO2 (CT) SCR (Selective Catalytic Reduction) catalyst. This study investigates the deactivation mechanism of Na on the Ce/TiO2 catalyst using Density Functional Theory (DFT) calculations, focusing on the adsorption of surface species and the pathway of elementary reaction. Due to the high affinity between Na atom and the catalyst surface, Na poisoning readily occurs on Ce/TiO2 (001) surface. The presence of Na on the catalyst surface hampers hydrogenation, oxygen vacancy formation, and NH3 adsorption, while also reducing the catalyst's acidity and reducibility. Additionally, Na impedes the NH3 dehydrogenation reaction, as well as the formation and decomposition of NH2NO and the dissociation of O2 on the catalyst surface, thereby inhibiting the NH3-SCR reaction. The inhibition of NO2 formation further affects the fast SCR reaction, which may explain the low-temperature deactivation of the catalyst.

Promotion effect of Co addition on the activity and SO2 tolerance of CrCe catalysts for selective catalytic reduction of NOx with NH3 at middle–low temperatures

AbstractBackgroundNitrogen oxides (NOx) can result in a series of environmental issues (haze, photochemical smog and acid rain) that endanger human health. Currently, selective catalytic reduction (SCR) of NOx with ammonia (NH3‐SCR) has been considered to be one of the most efficient techniques to eliminate NOx.ResultsNovel Co‐modified chromium/cerium (CrCe) catalysts were synthesized by the citric acid method to improve their insufficient SCR activity and sulfur dioxide (SO2) resistance for NOx removal at middle–low temperatures. Cobalt (Co) addition could significantly increase the catalytic activity of CrCe catalysts, and the 5CoCrCe catalyst exhibited the highest catalytic activity with ≤95% NOx conversion and >80% N2 selectivity within 180–280 °C. In addition, the 5CoCrCe catalyst exhibited outstanding SO2 resistance as well as reversibility, with >90% NO conversion under 600 ppm SO2 for 510 min.ConclusionIt was determined by characterizations that Co addition can increase SSA, Cr6+ concentration, electron transfer between Co and Cr by redox cycle (Cr6+ + 4Co2+ ↔ 4Co3+ + Cr2+) and the redox properties of the CrCe catalyst. The 5CoCrCe catalyst surface consists mainly of Lewis acid and its NH3‐SCR reactions follows the Eley–Rideal (E–R) mechanism. The 5CoCrCe catalyst exhibited outstanding SO2 resistance due to higher Cr3+, Ce3+ and Oα concentrations and strong redox cycles (3Ce4+ + Cr3+ ↔ 3Ce3+ + Cr6+ and Cr6+ + 3Co2+ ↔ 3Co3+ + Cr3+). Furthermore, in situ DRIFTS results revealed that SO2 could induce more adsorbed NOx and NH3 species instead of bulk sulfate, which also was responsible for good SO2 resistance. © 2023 Society of Chemical Industry.